To achieve high operational rates with small computers and microprocessor-based devices, it has been necessary to develop higher speed microprocessors and the peripheral circuits such as random access memories (RAM), read only memories (ROM) or display drivers. When a high speed microprocessor is utilized, the associated peripheral devices must be compatible in order to be addressed by the microprocessor. Normally, microprocessors address a peripheral device in one cycle of the microprocessor clock and then sample the response from the sampled peripheral device in the next clock cycle. This is a disadvantage in that the increased speed of the microprocessor requires that all of the peripheral devices must operate at the same high speed.
Although the use of high speed microprocessors and high speed associated peripheral devices results in higher operational speeds for certain functions, it also generally results in increased equipment costs. If the particular application requires high speed access to the information stored in RAM, this cost must be tolerated. However, there are some functions of a microprocessorbased device that do not necessarily have to be performed at high speed. Such operations include those that display information to a user. For example, it is desirable to perform computations very quickly, but the use of high speed display devices is often not necessary since the visual response of the operator is much slower than the rate at which the microprocessor can effect a display change. Therefore, it would be desirable to utilize a slower and less expensive peripheral device to provide the display function of the microprocessor-based device. However, since the response time or speed of the display is dictated by the speed and type of microprocessor used, the use of a lower speed and thus lower cost display is not normally possible.
One previous solution to this problem has been to introduce a wait or hold mode of operation into the microprocessor. In this mode, the microprocessor continues to operate at its maximum clock speed, but allows the addressing signal to maintain a particular state for a predetermined number of clock cycles. This allows sufficient time for a relatively slower peripheral device to respond. One type of device that incorporates this mode of operation is an 8086 Microprocessor manufactured by Intel Corp. However, this wait or hold operating mode is not universally available, thus requiring the use of a particular microprocessor which can substantially increase the cost of the microprocessorbased device.
In view of the above disadvantages, there exists a need for a microprocessor-based system that is of sufficient versatility to utilize both slower and less expensive peripheral devices and the more expensive high speed peripheral devices.